Electrical Diagram for a CSIR Motor

A start capacitor is connected in series with the start winding of the
CSIR motor to provide a larger phase shift when voltage is first applied
to the motor. The increased phase shift causes a stronger magnetic field
to pull on the rotor to cause it to begin to spin. The electrical diagram
for the CSIR motor connected for forward rotation and for reverse rotation
is provided in Fig. 1. The start capacitor is usually rated for over
100 microfarads (up to 1000 uF).

The start capacitor can provide large amounts of capacitance for a short
period of time, such as during starting. After the motor is started,
the capacitor must be removed from the circuit so that it won't overheat.
This is accomplished with the end switch. The electrical diagram in this
figure shows the capacitor connected in series with the start winding.
When voltage is first applied to the motor, both the start winding and the run winding will be energized. When the rotor reaches nearly full
rpm, the end switch will open and disconnect the start winding and capacitor
from the circuit. That is, no current will flow through either the start
winding or the capacitor and they can cool down and be ready for the
next time the motor is started. When the motor is de-energized and the
rotor slows to a stop, the end switch is closed again and the start winding and capacitor are reconnected to the circuit for the next start.

Since the start capacitor is physically mounted on top of the motor,
a metal cover is placed over it to protect it from damage. Since the
capacitor’s case is made of plastic, it can easily be cracked and damaged
if the motor is used in a harsh industrial environment.